The invention described herein was a result of funding by The National Institutes of Health, grant number GM-34548, (R. R. Birge, principal investigator).
1. Field of the Invention
This invention relates generally to molecular electronics and the use of photosensitive chemicals in digital optical memories and artificial retinas, and specifically relates to the photosensitive protein molecule, bacteriorhodopsin.
2. Discussion of the Related Art
Photochemical reactions are highly specific reactions with varied and specific products. In the biological field, the existence of microbes capable of utilizing light energy to drive the synthesis of cellular components has been known since the late 19th century. Many proteins in nature serve the function of transporting ions across some medium, such as transporting protons from the interior of the bacterial cell to the exterior in a cyclic process to create a proton current. The proton gradient is used to generate adenosine triphosphate (ATP), necessary for energy storage and cell synthesis. One such protein found in nature is bacteriorhodopsin, which is a relatively small (about 4000 atoms) membrane protein found in the membrane of Halobacterium salinarium. When struck by light, the protein changes its structure and transports a proton across the membrane, thereby supplying energy to maintain cell metabolism.
The photochemistry of bacteriorhodopsin is mediated by a retinal chromophore which is bound to the protein via a protonated Schiff base linkage to LYS216. Upon the absorption of light, bacteriorhodopsin converts from a dark-adapted state containing a mixture of 13-cis and all-trans chromophores to a light-adapted state which contains only the all-trans chromophore. The photocycle of light-adapted bacteriorhodopsin is responsible for the proton pumping capability of the protein.
The functioning of bacteriorhodopsin as an optical material requires the presence of Ca.sup.2+ and Mg.sup.2+ ions which are bound at specific binding sites both inside the protein structure and on the protein surface. It is believed that there are two high affinity binding sites and four low affinity binding sites at various locations about the molecule. Removal of the calcium and magnesium cations from the native bacteriorhodopsin purple membrane produces the blue membrane, a protein which lacks a photocycle and which does not pump protons. It is known in the art that the optical and photochemical properties of the protein can be restored by adding either calcium or magnesium to an aqueous solution of the blue membrane.
As described above, photochemicals have been identified, studied, and used in various industries. One area that is of particular interest is the use of bacteriorhodopsin in the computer industry. In the computer industry, computer memory is a limiting factor of today's computer technologies. Optical data storage, particularly three dimensional optical data storage, offers many advantages over conventional two dimensional technologies with respect to storage capacity, cost and access time. A discussion of optical memory storage and protein-based computers is contained in the article, "Protein-Based Computers", Scientific American, Vol. 272, No. 3, March 1995, which is incorporated herein by reference.
A three dimensional optical memory device utilizing a film of bacteriorhodopsin is described in U.S. Pat. No. 5,253,198 to Birge et al., owned by a common assignee and incorporated herein by reference. An optical random access memory utilizing photosensitive chemicals is described in U.S. Pat. No. 5,228,001 to Birge et al., also incorporated herein by reference. The device described therein provides a high-speed optical random access memory that can employ a laser of low to moderate power for write, read and erase operations.
One drawback in the use of bacteriorhodopsin is low quantum efficiency of the data write process, which is on the order of 10.sup.-4 for the O-P photochemical transformation. Another drawback is the low yield of the O intermediate in the native protein at ambient temperatures. Therefore, there is a need to increase the yield of the O state so that enhanced photochemical conversion into the P state can be realized.
Another use of bacteriorhodopsin is in artificial retinas. The use of bacteriorhodopsin in artificial retinas is described in the article "Protein-based Artificial Retinas," Chen, Zhongping and Birge, Robert R., Trends in Biotechnology, July 1993. One drawback of this use of bacteriorhodopsin is that the protein film poisons the semiconductor due to the leaching of the calcium and magnesium metal cations from the bacteriorhodopsin molecule into the semiconductor surface.